The present invention relates generally to a detector or sensing system, and more particularly to such a system which provides for rapid and efficient switching of the output spectrum of a molecular laser preferably arranged in a dual or tandem optical cavity configuration and utilizing a CO.sub.2 laser, a CO laser, or the like. In order to achieve the rapid wavelength switching available in the operation of this system, a Bragg cell is utilized in a secondary or tandem configuration with the main cavity of the system. The arrangement of the present invention provides for relatively low coupling losses, as well as rapid switching speeds, with switching times of the order of 1 to 10 microseconds being achieved.
In the present arrangement, a Bragg cell is disposed in a secondary cavity which is arranged in tandem to the main laser cavity. The arrangements proposed can be linear or ring laser configurations. The arrangements in a ring configuration preferably consist of a three-mirror resonator in the main laser cavity with a modified form of ring resonator preferably being utilized in the secondary or wavelength controlling feedback cavity. The components in the feedback cavity include one or more acousto-optic modulators operated in the Bragg cell mode, together with a reflector element, the reflector element normally being a mirror or a grating operated in the Littrow mode. One reflector element of the feedback cavity is mounted to a piezoelectric element with the arrangement providing on-resonance tuning of the secondary or wavelength controlling feedback cavity for the case of the ring laser and off-resonance tuning for the linear coupled cavities.
In the past, various attempts have been made to provide for rapid switching of lasers. For example, a number of prior U.S. patents provide examples of utilizing tunable lasers with Littrow gratings. For example, in U.S. Pat. No. 3,443,243 to Patel, a grating is located beyond the reflective elements defining the optical resonator cavity of a laser and light reflected from the grating passes through an aperture to maximize frequency resolution. The concave reflective grating employed in Patels' device can be rotated about an axis parallel to the grating lines. In U.S. Pat. No. 3,739,295 to Shah, a rotatable plane reflective grating is employed as a tuning element in a dye laser. An aperture is included between the grating and one of the resonator cavity reflector elements to block fluorescence of radiation returning from the grating to the lasing medium. In a patent to Comera et al, U.S. Pat. No. 4,241,318, a laser's plane reflector grating is adapted, in combination with a wheel containing two optical elements, to place the optical elements periodically in the path of the laser beam. This deflects the laser beam so that the angle of incidence of the beam on the grating is modified and a different wavelength is reflected back along the longitudinal axis of the laser for each element. The grating and wheel are rotatable as a unit relative to a plane perpendicular to the longitudinal axis of the laser so that more than two wavelengths can be selected.
U.S. Pat. No. 4,028,636 to Hughes granted June 7, 1977 shows an acousto-optic tuning system for an organic dye laser, the tuning element comprising a diffraction grating 20 and a Bragg diffraction cell driven by an RF responsive, ultrasonic transducer, all within the primary optical cavity. In U.S. Pat. No. 4,216,440 to Rahn et al granted Aug. 5, 1980, surface acoustic waves in the sides of a piezoelectric prism 13 provide two reflective diffraction gratings for tuning the resonant cavity of a laser 11.
An embodiment shown in FIG. 9 of U.S. Pat. No. 4,287,486 to Javan discloses a double grating arrangement with the gratings facing each other, albeit offset and not parallel, so different wavelengths of light from the laser are dispersed onto a mirror. The mirror is rotated to sequentially regenerate only one of a series of wavelengths at a time. The laser is triggered to fire when light of the first wavelength strikes the mirror in perpendiculr relationship, with the pulse continuing until all the wavelengths of interest are scanned. Thus a chirped pulse (i.e. a pulse with a change in wavelength within the pulse) is provided. Javan however, does not disclose a pulsed laser wherein each pulse can be tuned to a different wavelength, particularly if very fast switching times are desired.
In "CO.sub.2 Probe Laser with Rapid Wavelength Switching", S. Holly and S. Aiken, SPIE Volume 122, Advances in Laser Engineering (1977), rapid tuning of a continuous wave CO.sub.2 probe laser is provided by positioning eight gratings in carousel fashion about a mirror mounted on a scanner driven by a stepping motor. The eight gratings are switched in sequence into the optical cavity of the probe laser. Switching between wavelengths was reported to occur within approximately 10 milliseconds. The number of wavelengths which can be scanned by the Holly and Aiken device is limited by the number of gratings provided and the alignment problems require a complex electro optics control loop system.
Due to the many levels of the rotation-vibration band of CO.sub.2 between 80 to 100 lines of CO.sub.2 -laser transitions can be brought into oscillation. Cavities of relatively short length, such as in the range of approximately 6 inches, typically bring a substantial number of wavelengths or lines into oscillation, such as from 10 to 12 different wavelengths. On the other hand, cavity lengths of from between 2 feet and 3 feet may only bring one wavelength into oscillation.
In CO.sub.2 laser structures of relatively long cavity length, the predominant output frequency is a wavelength of 10.591 microns, with this wavelength being known as the P(20) line. If, for a system employing such a laser, oscillation at wavelengths other than the P(20) line is desired, means are normally provided in the system to provide a selectable wavelength for the system output. The arrangement of the present invention provides for off-resonance tuning in the auxiliary cavity of the coupled cavity configuration, thus achieving its goal of rapid wavelength switching in the main cavity at comparatively low power levels in the auxiliary cavity.
By way of contrast with the prior art, the present invention provides a relatively simple but highly efficient system which has been developed for providing an output which rapidly scans a number of wavelengths which are derived from a single laser source, with the wavelength scanning occurring at a rapid rate, such as rates on the order of from 1 to 10 microseconds. It has been found that such a system would be particularly useful in spectrographic analyses, such as for use in combination with diagnostic laboratory instruments, remote sensing systems for detection and determination of certain toxic gases or the measurement of air-borne pollutants, and also in military laser systems where immunity against counter-measures is important.